The natural gas industry and large volume consumers are seeking inexpensive methods for real-time monitoring of natural gas (NG) constituents. Such real-time monitoring is especially needed for pipeline switching and metering stations, turbine generators, and other industrial user sites.
For pipelines, it is important to have information on the composition of NG in the main pipeline and of the blending in at every mixing location. When the fraction of heavier constituents is too high, it becomes unsuitable for certain end users. Heavier constituents may also condense where temperature and pressure drop, such as at meters, valves, and various locations in end-user facilities. Therefore, speciation is necessary to obtain the composition and hydrocarbon dew point of the NG.
It is also desirable by natural gas sellers and buyers to have immediate knowledge of the BTU value for equitable trade (a BTU, or British Thermal Unit, is the amount of heat required to raise the temperature of one pound of liquid water by 1° F. at its maximum density). Gas heating value and flow measurement errors on the order of a percent or less can have significant impacts to the industry in the areas of lost revenues and cash flow, since many pipeline networks possess gas transfer capacities that range from 0.5 to 1 billion cubic feet per day. As natural gas is sold by the BTU (i.e., the heating value), improved analysis will produce more equitable trade between buyers and sellers by reducing unaccounted for gas content. Because instrumentation for accurate natural gas analysis is expensive to purchase, operate and maintain, ubiquitous metering devices provide only volumetric measures of the quantity of natural gas consumed. However, final billing is determined a month or more later when information from lab analysis is used to convert the volumes to BTU heating values. This difference, known as unaccounted for (UFA) gas content, can be reduced by improved sample handling and analysis, producing more timely and accurate billing. A further complication arises during gas transfers between wholesale or commercial entities along the gas supply chain when both gas volume and heating value indices can be used in the transaction. Discrepancies between the two can result in billing errors or lost revenues. Therefore, a gas transfer system based solely on heating value and made possible by a low-cost approach would be beneficial to the natural gas industry.
Further, certain industrial users have a strong interest in knowing the composition of incoming natural gas. Industrial users will benefit through continuous feedback of physical gas properties during consumption, in particular with gas turbine engines used for power and electricity generation. For NG fueled turbines, efficiency of power generation is dependent on fuel properties and the temperature of the combustion cycle. Turbine controls are based on models that indirectly calculate peak combustor temperatures based on estimates of the fuel heating value. Inaccuracies as high as 40° C. limit the ability to operate at the optimum firing temperature. Controllers error on the side of safety to prevent part-life degradation from overfiring the turbines. Even a small increase in operational efficiency of natural gas-fueled power can save large amounts of natural gas per year and significantly reduce related nitrogen oxide emissions.
Finally, a low-cost monitoring device would enable distributed sensing of the natural gas infrastructure, providing greater pipeline security. Improved real-time information on the composition of gas throughout the system would allow early detection of deleterious events such as pipeline leakage or fouling.
The standard method for natural gas BTU analysis uses gas chromatography (GC) to speciate fuel flows, and then infers heating value from this speciation. See ASTM International 2003, “Standard Test Methods for Analysis of Natural Gas by Gas Chromatography,” D1945-03. This method is highly accurate, but expensive in terms of equipment, maintenance, operation, and personnel. Samples are often collected into bottles for once-a-month lab analysis, resulting in billing accuracy delays of a month or longer. In addition, standard GCs and detectors require pressurized specialty gases, some of which are flammable. While slightly less expensive field-portable devices are becoming available, detector requirements have not changed. Automated on-line GCs are currently too expensive to widely distribute over the pipeline infrastructure for real-time analysis. Therefore, on-line GCs are only used at large custody transfer stations where accounting delays would become more costly. Under present conditions, the limited number of instrumentation locations within the gas pipeline network results in significant errors in mass balance and cost recovery that directly influence custody transfer operations. A number of research organizations are attempting to perfect inferential techniques using thermal conductivity or speed of sound measurements. Inorganic compounds such as carbon dioxide, carbon monoxide, and nitrogen confound these measurements, so their concentrations must be independently determined. So far, these techniques cannot determine nitrogen concentration without resorting to gas chromatography.
A pellistor is a type of calorimetric combustible gas sensor that detects a change in the temperature of a heated catalytic element when exposed to a mixture of combustible gas and air. Pellistors in the past have not been designed for speed. Most take several minutes to make accurate readings due to their large thermal masses and the thick diffusion-limiting layers of catalyst substrate on the surface of the sensing elements. Recently, much faster micropellistors, or microcalorimeters, have been developed for determining BTU heating values of natural gas and other fuel gas streams. See U.S. Pat. No. 6,786,716 to Gardner et al.; M. Moorman et al., “Microcombustor array and micro-flame ionization detector for hydrocarbon detection,” Proc. SPIE 4981, 40 (2003); M. Moorman et al., “Lower Heating Value Sensor for Fuel Monitoring,” submitted to IEEE Sensors (2005); and P. N. Bartlett and S. Guerin, “A Micromachined Calorimetric Gas Sensor: an Application of Electrodeposited Nanostructured Palladium for the Detection of Combustible Gases,” Anal. Chem. 75, 126 (2003); which are incorporated herein by reference. The microcalorimeter typically comprises a thin catalyst layer deposited on a thin, resistively heated surface. The fuel, premixed with air, catalytically combusts on this surface. The heat of combustion is measured directly from a feedback circuit powering the sensor. This rapid and sensitive sensor provides a direct measurement of the BTU content of the fuel. When combined with a density measurement, the Wobbe Index can be determined, which is an important fuel property used to aid combustion control in systems such as gas turbines. This microcalorimeter provides stand-alone gas monitoring where low precision analysis (+/−5% BTU) is acceptable. However, while simple and inexpensive, this sensor alone does not provide direct information on constituents of the NG stream.
For many users, including gas transfer/mixing stations and electricity generating turbines, accuracy of 0.1% BTU is required. Further, for pipelines and turbines, speciation is necessary to obtain the NG composition and hydrocarbon dew point. Especially where large quantities of NG are consumed (medium-size or larger switching stations, gas-turbine electricity generators), all fuel properties need to be known. In these applications, widespread deployment of advanced monitoring devices is highly desirable. Further, for market acceptance, the monitoring device must be of low cost. Deployment of low-cost devices within the pipeline network would translate to increased revenues for the gas industry by virtue of more detailed information on the heating value of gas within the system at any given time. Industrial end users would benefit by running natural gas fueled turbines closer to design limits, safely maximizing power while maintaining pollution control.